The major objective of the present research proposal is to determine the factors which influence the distribution of cortical projection neurons. Anatomical experiments utilizing retrograde tracers (horseradish peroxidase or fast blue) are planned to determine the distribution of the cells of origin of cortical projection neurons after neonatal removal of afferent input or of the targets of cortical projection neurons. Other experiments will utilize horseradish peroxidase as an anterograde label to visualize developing callosal fibers in the rat somatosensory cortex. The experiments are designed to test the hypothesis that cortical projection classes are divided into several classes between which there is little developmental interaction and that each class is initially a homogeneous population of neurons which only later become associated with a particular target site. The proposed research will add to our understanding of normal developmental processess and of anomalies which result from their disturbance.